The present invention relates to an internal combustion engine, especially for a portable, manually-guided implement such as a power chain saw, a cut-off machine, a brushcutter, a trimmer, or the like, and has a combustion chamber that is formed in the cylinder of the engine and that is delimited by a reciprocating piston that drives a crankshaft that is rotatably mounted in a crankcase. A fuel/air mixture prepared in the Venturi section of a diaphragm carburetor is supplied to the internal combustion engine. The air portion of the mixture is supplied to the Venturi section via an intake channel, and the fuel portion of the mixture flows to the Venturi section via a main nozzle path that branches off from a fuel-filled control chamber that is supplied with fuel via a fuel line and a feed valve, which is controlled by a control diaphragm that delimits the control chamber.
An engine of this general type is known from DE 199 00 445 A1. The fuel/air mixture is drawn into the crankcase and, as the piston moves downwardly, is conveyed into the combustion chamber via transfer channels. To reduce the scavenging losses, in particular the transfer channels that are disposed close to the exhaust communicate via diaphragm valves with air channels that supply clean air, so that the rich mixture is shielded from the exhaust by fuel-free air that flows in a contemporaneous manner. This known engine can be operated as an engine having scavenging collection or also as an engine having charge stratifying, and exhibits a very good exhaust gas characteristic at low fuel consumption.
Because of the system, the mixture becomes leaner under full load and dropping speed, since in such an operating state an over proportional amount of fuel-free air is drawn in via the bypass air channels. The engine becomes starved, and its power drops.
It is therefore an object of the present invention to provide an improved internal combustion engine of the aforementioned general type that even at a speed that drops under full load ensures a complete combustion with a powerful output.